The solar wind termination shock marks the inner boundary of the heliosheath, the outermost layer of the expanding solar atmosphere that is strongly perturbed by the interaction with interstellar medium. Voyager 1 will be at 96 AU in June 2005 and has been in the vicinity of the shock since reaching 85 AU in mid-2002. During most of this time, Voyager 1 has observed two long episodes of a new low energy component, suggesting this is a durable feature of this region of the heliosphere. The energetic ions stream outward along the magnetic field, placing their source radially inward of Voyager 1, even though there is no indication in the magnetic field that Voyager 1 has crossed the termination shock. During this time, the higher energy anomalous cosmic rays continue to be strongly modulated, so they must have a different source region on the shock that is not magnetically connected to Voyager 1. A new episode of the low energy component appearing in late 2004 differs from the earlier episodes, having much reduced variations in intensity. These and other observations reveal a complex shock region and pose a number of questions that continue to stimulate new analyses and interpretations

The Voyager 1 and 2 spacecraft are now approaching the solar wind termination shock and will soon enter the high temperature low velocity region of the heliosphere separating the solar wind from the local interstellar medium. Physical properties of the heliosheath have been the subject of numerous theoretical investigations which are now awaiting experimental verification. In this review we will discuss recent theoretical model predictions for the dynamical evolution of partly ionized plasma flows and highly energetic particle populations in the heliosheath under the influence of a variety of processes of solar and interstellar origins. Three principal sources of variability are considered: (a) the 11- and 22-year solar cycle, (b) large-scale disturbances transmitted through the terminations shock, and (c) macroscopic instabilities at shear and density interface layers. Sunspot cycle variations of the solar wind dynamic pressure force the entire heliosphere to undergo "breathing", where the termination shock and the heliopause move asymmetrically toward or away from the Sun. Possible effects of the termination shock oscillations on anomalous cosmic ray intensities upstream of the shock will be briefly addressed. Solar cycle-driven heliosheric magnetic field reversals are expected to generate a complicated magnetic field topology in the heliosheath, with important consequences for galactic cosmic ray modulation. In this context we will discuss the possibility of enhanced attenuation of galactic radiation by the heliosheath and the "modulation wall" bordering the heliopause. Global heliospheric disturbances propagating outward with the solar wind collide with the termination shock and inject waves and discontinuities into the downstream region adding to the turbulent content of the heliosheath flow. Remarkably, the oscillations in the outer heliosphere can persist for several years following a collision. Finally, the heliopause itself, as a tangential discontinuity separating a dense and cold interstellar flow from the tenuous hot heliosheath plasma is subject to hydrodynamic instabilities of the Rayleigh-Taylor and Kelvin-Helmholtz types. Using theoretical analysis and computer simulations we predict that charge transfer between the plasma and interstellar hydrogen atoms would drive the interface unstable. The role of secondary atom populations and magnetic fields on linear and nonlinear instability development are also discussed.

A simple, MHD-based, model has shaped our expectations on the boundary between the solar wind and interstellar space. Predictions of the location of the termination shock (TS) in particular have ranged over the years from ~ 5 to > 100 AU, depending on the choice of parameters inserted into the balance equation. It is now clear that Voyager has reached the vicinity of the TS at ~85-95 AU, and may have crossed (Krimigis et al, 2003) or still upstream but close (McDonald et al, 2003). In either case, the observations are at substantial variance with many, if not all, the expected characteristics. (a) Solar wind convection is no longer dominating flows in this region; instead, intense anisotropic beams of ions, apparently field-aligned, are flowing tagentially to the radial direction and dominantly outward away from the sun. (b) Relativistic electrons are present concurently with the ions, and exhibit similar intensity-time profiles. (c) Ion composition is characteristic of anomalous cosmic rays (ACR) but spectra are power laws at lower (< 50 Mev total energy) enegies and still exhibit a «hump» at ~100 Mev, an ACR characteristic. (d) Galactic cosmic rays (GCR) no longer show the expected solar cycle variation at V1, and recent (March 05) V2 GCR intensities are nearly at the same level as those at Voyager 1. (e) The interplanetary magnetic field, contrary to previous examples in the vicinity of interplanetary shocks, appears to be insensitive to the low energy particle activity (Burlaga et al, 2003), but is viewed as weakly corelated to GCR variations. (f) Beam anisotropies preclude a diffusive TS, contrary to expectations, although the power law spectrum is consistent with shock predictions. Updated observations will be presented and implications will be discussed in the context of current models.

Voyager 1 observations in the outer heliosphere impose important constraints on the physics of particle acceleration at the heliospheric termination shock. The large enhancements of energetic particle intensities observed in 2002 and 2004 provide new and important information. The observed spectra, composition and anisotropies of the energetic particles have have proved challenging to interpret in terms of a consistent physical model. We demonstrate that the direction and magnitude of the three-dimensional, vector anisotropies (radial, azimuthal and latitudinal) depend in an important way the large-scale structure of termination shock. In particular, the shape of the shock is probably not spherical, both globally and on a small scale. The termination shock probably has a blunt shape, as suggested by simulations and straightforward physical arguments, and possibly is also distorted asymmetrically by the very local interstellar magnetic field, as suggested by recent observations of interstellar neutrals. Implications of these for the nature of the interaction of the heliosphere with the local interstellar medium, for the local interstellar magnetic field and for the acceleration of energetic particles will be discussed. The observed nature and direction of the three-dimensional accelerated-particle anisotropies seem to be understandable in terms of a non-spherical, asymmetric large-scale structure such as that suggested by observations, simulations and theory. The intensities and energy spectra may also be understandable in terms of more local fluctuations of the shock and magnetic field.

Results of our new multi-component model of the solar wind interaction with the local interstellar medium are presented. In the frame of this model the solar wind protons, electrons and pickup ions are considered as separate components. The solar wind protons and electrons are treated as co-moving fluids, while the pickup proton component is described by a Fokker-Planck equation for the isotropic velocity distribution function. Both solar wind protons and pickup protons interact with the interstellar H atoms by charge exchange. In the model we solve kinetic equation for the interstellar H atom component by Monte Carlo method self-consisitently with the set of kinetic and hydrodynamic equations for the plasma components. Detailed mathematical formulation and assumptions of the model and its first results on pickup protons and heliospheric ENA fluxes are presented.

Results are presented of a numerical investigation of the three-dimensional interaction of the solar wind (SW) with the local interstellar medium (LISM) while taking into account the effects of coupling between the interstellar and interplanetary magnetic fields (ISMF and IMF). In contrast to other theoretical MHD models of the heliospheric interface, we treat neutral hydrogen atoms, which experience charge exchange with the hydrogen plasma, self-consistently, in the framework of the multi-fluid approach. The effect of the IMSF strength is analyzed in superfast and sufast, superslow interaction regimes. It is shown that, depending on the ISMF direction with respect to the LISM velocity and solar ecliptic plane, the heliospheric current sheet bends into one of hemispheres. As a result, the flow variable distributions become substantially asymmetric with respect to the ecliptic plane. Reasons are discussed of the origin of V-shaped grooves on the surface of the heliopause for certain implementations of the boundary conditions in the inner heliosphere. The distribution of the magnetic field on the outer side of the heliopause is analyzed for the ISMF orientation that is believed to be responsible for the distribution of the 2--3 kHz radio emission sources. Unsteady phenomena are investigated related to the propagation of global merged interaction regions through the three-dimensional heliosphere.

The outer heliosphere, and the termination shock in particular, produces X-ray emission. This emission is due to the charge exchange process between highly ionized minor ions of the solar wind interacting with neutral hydrogen atoms from our local interstellar medium. In this work, we combine high-resolution 2D magnetohydrodynamic simulations with an X-ray emission mechanism in order to produce two-dimensional X-ray brightness maps of the heliosphere as seen from the outside. The model is capable of tracing the full evolution of solar wind ion species along the solar wind stream lines, and thus correctly treats both the collisionally thin and collisionally thick regimes. X-ray emission from charge transfer should also occur in the astrospheres of other stars, so that this technique may ultimately be used to probe the properties of stellar winds of nearby stars.

We have modeled SOHO-SWAN Hydrogen cell observations, and found that the interstellar neutral hydrogen flow direction differs by about 4 degrees from the neutral helium flow direction, which has been recently derived with an unprecedented accuracy using combined data sets. We discuss possible explanations for this deviation. The most likely explanation is a distortion of the heliospheric interface under the action of an interstellar magnetic field, not aligned (nor perpendicular to) the direction of motion of the solar system through the local interstellar cloud. Charge-transfer reactions between H atoms and ions propagate the departures from axisymmetry of the plasma flow to the neutrals. In this case constraints on the direction of the ambient interstellar magnetic field can be derived from the observed deviation. The data imply that Voyager 1 is heading towards the less compressed region of the interface.